1. Field of the Invention
The field of the present invention is screw type pumps, such as mechanical superchargers, having a pair of screw rotors meshed with each other and rotatably supported in a housing, and particularly, screw type pumps with a variable internal compression ratio.
2. Description of the Prior Art
Screw type pumps with a variable internal compression ratio are already known, for example, from Japanese Utility Model Application Laid-open No. 46435/89 and Japanese Utility Model Publication No. 40727/78.
In a mechanical supercharger in the form of a screw type pump that is disclosed in Japanese Utility Model Application Laid-open No.46435/89, a slide valve movable axially of the screw rotors is provided in the housing, so that the internal compression ratio is varied by driving the slide valve. Because the slide valve is moved axially, however, the size of the housing is increased by the range of such movement. Moreover, in a mechanical supercharger using screw rotors, the temperature varies in the axial direction of the supercharger. Therefore, in a structure in which the slide valve is moved in the axial direction, a difference in clearance between the slide valve and the housing is produced in such axial direction due to a difference in thermal expansion depending upon the variation of temperature, thereby making reliable sealing difficult.
In a supercharger in the form of the screw type pump disclosed in the above-identified Japanese Utility Model Publication No.40727/78, a portion of the intake gas is circulated by controlling an opening provided in the side of a housing for opening and closing thereof and therefore, a reduction in efficiency occurs.